Stainless steel and PWHT – Post weld heat treatment

Most austenitic stainless steel weldments do not require post-weld heat treatment. For those that do, the heat treatment temperature should reflect the particular concern that is being addressed.

The necessity for any type of heat treatment of austenitic chromium-nickel steel weldments depends largely on the service conditions encountered. For some applications, heat treatment is used to impart the greatest degree of corrosion resistance possible, eg by solution treating to homogenize the composition or stabilizing, to minimize the risk of sensitization during subsequent elevated temperature exposure.

In other applications, heat treatment may be used for stress relieving. This may give more dimensional stability for components to be machined or reduce stress-corrosion cracking (SCC)/reheat cracking susceptibility. For dimensional stability, a fairly low-temperature treatment at around 400°C, giving partial stress relief, may be sufficient. For SCC resistance it may be appropriate to use a much higher temperature treatment, at around 1000°C, so that most of the internal stress is relieved. Heavy sections in certain stainless steels (e.g. stabilized grades such as type 347) operating in the creep temperature range can suffer reheat cracking, and stress relief (>950°C) is the only guaranteed method of avoiding this problem.

For full solution annealing (>1000°C), if rapid cooling through part of the cooling cycle is required to avoid unacceptable precipitation, it should be noted that this will create new stresses of considerable magnitude. In such circumstances, a lower temperature treatment (eg at around 400°C) giving partial stress relief without precipitation of second phases may be preferable.

The need for a stress-relief treatment in weldments constructed of austenitic stainless steel has been over-emphasized in the past and has been removed from many standards.

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